The present invention relates to a coating resin composition, which has a high safety, provides a coating excellent in water resistance and solvent resistance, and is capable of easily and directly coating, particularly, polyolefin resin moldings; a coating resin composition therefor; and a method for coating with it.
Exterior parts of motorcars made of resins instead of metals or glass have come to be widely used. The use of resin moldings for forming, for example, bumpers, door mirror covers, moldings and spoilers is now increasing. Polyolefin resin materials are becoming to be used in a larger amount than urethane resins for forming such resin moldings for economical reasons. In addition, the polyolefin resins are excellent in chemical resistance, water resistance and moldability.
When such a polyolefin resin material is used, it is under more restrictions than metal coatings. For example, since the polyolefin resin material has a low polarity, the resultant coating has a poor adhesion. Another disadvantage of the polyolefin resin material is that since its thermal resistance is low, the coating must be heat-dried at a temperature of, for example, below 140.degree. C.
Investigations have been made on various methods for improving the adhesion of the coating to the polyolefin resin moldings. They include, for example, a method wherein the surface of the polyolefin resin molding is pretreated by a flame treatment, corona discharge, plasma treatment, treatment with chromium sulfate or the like, and then the top-coating is conducted; and a method wherein the surface of the polyolefin resin molding is cleaned with a solvent such as trichloroethane, then the surface is coated with a primer having a high adhesion and the top-coating is conducted. However, the satisfactory adhesion is not always obtained by these methods and, in addition, these methods have problems in that they are complicated because of the large number of the steps and that the cost is high, since the specified expensive primer is used.
For improving these techniques, a method for improving the adhesion to the polyolefin resin molding by using a coating composition containing a chlorinated polyolefin resin has been proposed. Various coating compositions were proposed. They include, for example, (1) coating compositions comprising a mixture of a chlorinated polyolefin resin and acrylic resin Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No. Sho 57-200438, (2) coating compositions comprising a copolymer of a chlorinated polyolefin resin and acrylic monomer and, if necessary, a polyester prepolymer having a number-average molecular weight of 500 to 3,000, combined with an isocyanate compound (J. P. KOKAI Nos. Sho 59-27968, Sho 62-149734 and Hei 4-132783), (3) coating compositions comprising a combination of a copolymer of chlorinated polyolefin resin/acrylic monomer/chlorinated polydiene, with an isocyanate compound, and (4) coating compositions prepared by incorporating a hardener such as an isocyanate prepolymer or melamine resin into a mixture of a chlorinated polyolefin resin and an acrylic resin or into a product obtained by grafting an acrylic monomer onto a chlorinated polyolefin (J. P. KOKAI No. Hei 5-117574).
However, the coating composition (1) has defects in that the chlorine content thereof must be controlled to 35% by weight or below and in that, since the coating composition is easily divided into two phases, the properties such as the dispersion stability of the pigment, gloss of the coating, adhesion of the coating, solvent resistance and weather resistance are insufficient. Although the stability of the coating composition (2) is higher than that of the coating composition (1), the coating composition (2) has defects in that the gloss and weather resistance of the resultant coating are insufficient and in that the coating composition has problems of handling and safety, since the isocyanate compound has a strong toxicity. The coating from the coating composition (3) has insufficient solvent resistance and weather resistance and the coating composition has the same problems of handling and safety as those of the coating composition (2), since it also contains the isocyanate compound. The coating composition (4) has defects in that, when the isocyanate prepolymer is used, a problem of toxicity is caused and in that, when the melamine resin is incorporated, the water resistance and solvent resistance are reduced, since the hardening is insufficient.
Although various improved coating compositions have been thus proposed, they are yet insufficient in the field wherein a particularly high durability is strictly required as in the field of coating compositions for bumpers of motorcars. Therefore, they are used in only limited fields.
Under these circumstances, the development of a coating composition excellent in the handling properties and safety, capable of being cured at a low temperature and also providing a coating excellent in water resistance, solvent resistance and adhesion to the polyolefin resin moldings has been eagerly demanded.